As the integration degree of a semiconductor integrated circuit increases, there is a need for forming a fine pattern which cannot be formed by the conventional I-line (λ=365 nm), KrF (λ=248 nm), and ArF (λ=193 nm) exposure techniques. For example, as a dynamic random access memory (“DRAM”) having a memory capacity of more than one gigabit has been developed, it is needed to produce a fine pattern having a higher resolution. To produce the fine pattern, various Next Generation Lithography (NGL) techniques have been actively studied, and Extreme UV lithography (EUVL) is developed as one of the NGL techniques.
Generally, in the photolithography process for manufacturing a semiconductor, a photoresist composition works as follows. (a) A photoresist layer formed on a semiconductor substrate is exposed to an exposure light through a photomask (reticle) on which a semiconductor circuit design is engraved, and thereby a latent image of the photomask is projected to the photoresist layer; (b) the photoresist layer to which the latent image of the photomask is projected, is baked to activate acids in the exposed parts; (c) a main chain or a functional group of a matrix photosensitive polymer for photoresist is depolymerized or deprotected, or a matrix photosensitive polymer is cross-linked; (d) accordingly, the difference of solubility for a developer between the exposed and the unexposed parts increases; and (e) a photoresist pattern is formed by successive processes such as a developing process.
To form a fine photoresist pattern whose line width is less than 0.25 μm, deep ultra violet light of a short wavelength of less than 250 nm is used as the exposure light in the lithography process, and KrF (λ=248 nm) or ArF (λ=193 nm) excimer laser is used as the exposure light source. The photoresist composition should (a) have a good transparency for the exposure light; (b) have a good adhesiveness to a substrate; (c) have a good etch resistance; (d) cause no damages or defects such as Line Edge Roughness (LER), top loss, slope in the formed photoresist pattern; (e) be easily developed in a conventional developer, for example 2.38 weight % tetramethylammonium hydroxide (TMAH) solution; and (f) have a good thermal stability. The resolution or photosensitivity of the photoresist does not much effected by the thermal properties of the photoresist, but the thermal properties of the photoresist are important properties in handling the photoresist during the lithography processes. For example, the produced photoresist pattern is used as a mask in processes such as etching, ion injecting. Accordingly, the developed photoresist pattern should be reinforced by a thermal treatment so as to endure the severe conditions of the etching or ion injecting process. When a photoresist pattern is heated in a high temperature, the polymer resin of the photoresist is cross-linked, and then molecular weight of the polymer increases, and chemical resistance and thermal resistance of the polymer improves. However, when a photoresist pattern is heated in a very high temperature or the thermal property of the polymer resin is not good, the photoresist pattern collapses or flows before being hardened.